Umpty Um was the "noise of the monsters" according to a fictional version of the Doctor in the mind of David Karpagnon.
The Doctor said "they're the worst. Nothing scares me like the Umpty Ums." (PROSE: The Terror of the Umpty Ums [+]Error: Code 2 - no data stored in variables, cache or SMW.)
Behind the scenes[]
Umpty Um is an onomatopoeia of the "umpty-um umpty-um, umpty-um umpty-um" of the Doctor Who theme.